<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body >

	  
	  <form name="Dem">
	  
              <input type="button" size="8" name="dongho" readonly>
             
	  </form> 
<script> 
     var phut = 0
     var giay = 4
    function DemNguoc(){ 
     if(giay<=0){
        phut -=1
        giay = 60
     }
     if (phut<=-1){ 
        giay=0
        phut+=1 
            HetGio()
     } 
     else {
        giay-=1 
        document.Dem.dongho.value = phut+":"+giay 
        setTimeout("DemNguoc()",1000) }
    }

    function HetGio(){
            alert("Het Gio, goi submmit ");
    }
    DemNguoc() 
</script> 

</body>
</html>
